Development of Morphologically engineered Flower-like Hafnium-Doped ZnO with Experimental and DFT Validation for Low-Temperature and Ultrasensitive Detection of NOX Gas

Substitutional doping and different nanostructures of ZnO have rendered it an effective sensor for the detection of volatile organic compounds in real-time atmosphere. However, the low selectivity of ZnO sensors limits their applications. Herein, hafnium (Hf)-doped ZnO (Hf-ZnO) nanostructures are developed by the hydrothermal method for high selectivity of hazardous NOX gas in the atmosphere, substantially portraying the role of doping concentration on the enhancement of structural, optical, and sensing behavior. ZnO microspheres with 5% Hf doping showed excellent sensing and detected 22 parts per billion (ppb) NOX gas in the atmosphere, within 24 s, which is much faster than ZnO (90 s), and rendered superior sensing ability (S = 67) at a low temperature (100 °C) compared to ZnO (S = 40). The sensor revealed exceptional stability under humid air (S = 55 at 70% RH), suggesting a potential of 5% Hf-ZnO as a new stable sensing material. Density functional theory (DFT) and other characterization analyses revealed that the high sensing activity of 5% Hf-ZnO is attributed to the accessibility of more adsorption sites arising due to charge distortion, increased oxygen vacancies concentration, Lewis acid base, porous morphology, small particle size (5 nm), and strong bond interaction amidst NO2 molecule with ZnO-Hf-Ovacancy sites, resulting from the substitution of the host cation (Zn2+) with doping cation (Hf4+).


INTRODUCTION
Currently, the major atmospheric pollutants are oxides of nitrogen (collectively termed NO X ), which cause smog, acid rain, and respiratory diseases. 1−4 To tackle this challenge, accurate detection of NO X using stable and selective gas sensors is paramount. Among various gas sensing metal oxides (MOS: SnO 2 , TiO 2 , etc.), ZnO is significant because of its NO X detection ability with high sensitivity and selectivity. 5−7 However, high-temperature (over 200°C) operation is a major weakness of MOS sensors. 8,9 By changing the microstructure, morphology, or enhancing defects, the performance of ZnObased sensors can also be effectively improved, which can be achieved by engaging several synthesis methods, 10 UV illumination, or employing dopants. 11 Doping of semiconductor materials with transition metals (TM: Al, Cr, Sn, Mn, Ni, Co, Fe, and Cu) and rare earth metals (La and Tb) has been extensively used to enhance gas sensing properties 12,13 owing to lattice distortions in the host lattice, surface defects, especially the generation of oxygen vacancies, surface morphology variation, and grain size refinements. Transition metal hafnium (Hf) occurs in a different oxidation state (+4), resulting in the charge mismatch between Hf 4+ and Zn 2+ and possesses an ionic radius similar (0.78 Å) to Zn (0.74 Å). 14,15 Also, it has a low electro-negativity (1.3) and high basicity, which makes Hf a favorable candidate for doping ZnO to be used as a NO X sensor application. Gas sensing activity of metal oxide nanoparticles increases as a result of oxygen vacancy enhancement. Charge transfer between ZnO NPs and doped TM ions generates oxygen vacancies, alters neighboring cations valence states, and forms a donor state within the band gap. 16 Thus, enhanced gas responses from ZnO are possible by doping it with TM, which can have charge mismatch. 17,18 Furthermore, NO 2 , being acidic in nature, is easily affected by Hf (basic), resulting in a substantial interaction between Lewis acid and base 19 amidst the metal oxide and NO X molecules, thereby impacting the overall superior gas sensing response.
Another point to be considered in the case of doping is the critical doping concentration of the dopant into the host lattice, where at a high doping concentration (above the critical point), hafnium aggregates on the surface of ZnO, generating surface roughness. 14,20 Studies of the luminescence properties of Hf-ZnO have shown that hafnium induces defects (green emission) that are associated with oxygen vacancies. 20,21 However, to our knowledge, the sensing properties of Hf-ZnO for NO X detection have not been reported previously. Thus, it will be interesting to explore the effect of different hafnium doping concentrations on the properties of host metal oxides and their corresponding gas sensing behavior.
Herein, we investigated the role of hafnium doping on the morphology, microstructure, and defects modulation of the ZnO microsphere and thereby studied the effects of Hf doping on the improvement of ZnO-based NO X gas sensor, in terms of selectivity, stability, and fast response toward the target NO X gas. We fabricated various concentrations of Hf-doped ZnO (1, 3, 5, 7, 10% Hf) porous microsphere-based NO X sensors by adapting our previously reported hydrothermal synthesis strategy 22 for developing ZnO microspheres, which involved annealing the zinc hydroxide carbonate precursor in a vacuum environment to produce highly defective and porous microsphere for high NO X sensing. The correlation between different dopant concentrations toward oxygen vacancies and the properties of the sensing materials were investigated by employing various characterization techniques (X-ray diffrac-tion (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS)) and theoretical modeling using density functional theory (DFT). We established a comparative gas sensing behavior between the pristine and doped ZnO-based gas sensors. The excellent sensing response (S = 67) of the 5% Hf-ZnO sensor toward a very low concentration (0.8 ppm) of NO X gas at a low temperature (100°C) was obtained, which was further investigated to understand the mechanism behind such behavior.

MATERIAL AND METHODS
2.1. Fabrication of Hf-ZnO-Based Gas Sensors. All of the chemicals purchased from Merck are of analytical grade and have been used as received without further purification or modification, unless mentioned otherwise. The sensing materials employed here have been prepared via our previously reported work. 22 In brief, zinc nitrate hexahydrate (Zn(NO 3 ) 2 · H 2 O, 3 mmol), urea (CO(NH 2 ) 2 , 4 mmol), and trisodium citrate (Na 3 C 6 H 5 O 7 , 0.3 mmol) were dissolved in 100 mL of de-ionized water (DIW) to make a solution, which was then further prepared with different aqueous solutions of hafnium IV chloride (3−10 wt %) via constant vigorous stirring to form Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article a transparent solution. The resultant mixture was then transferred inside a 100 mL Teflon-lined autoclave and was maintained at 120°C for 5 h and then allowed to cool automatically to room temperature. The white precipitate collected was then taken for several washing steps with DIW and ethanol via centrifugation (5000 rpm, 15 min) followed by drying in air (80°C for 10 h). The resultant powder was annealed in a vacuum (6 × 10 −5 torr) furnace at 500°C (10°C /min) for 2 h (Figure 1). The synthesized materials with 0, 1, 3, 5, 7, and 10 wt % Hf doping are labeled as ZnO, 3% Hf-ZnO, 5% Hf-ZnO, 7% Hf-ZnO, and 10% Hf-ZnO, respectively, with other experimental parameters remaining constants. The pristine ZnO was prepared with the above-mentioned process without addition of any hafnium content.
2.2. Material Characterization. The detailed microstructural analyses of the as-synthesized sensing materials were executed by X-ray powder diffraction (XRD phase analysis using Bruker D8 Advance diffractometer with Cu Kα radiation), field emission scanning electron microscopy (FE-SEM morphology analysis using JEOL, JSM-7600F), highresolution transmission electron microscopy (HR-TEM microstructural analysis using JEOL, JEM-2100F, 200 kV), Brunauer−Emmett−Teller and Barrett−Joyner−Halenda (BET−BJH surface and pore analysis using BEL, Belsorp-Mini II) analysis, photoluminescence spectroscopy (PL, defect analysis using a Scinco fluorimeter FS-2, Xe excitation at 350 nm), and X-ray photoelectron spectroscopy (XPS, for chemical state and defects analysis using a Thermo Scientific, ESCALAB 250Xi spectrometer, 1450 eV monochromatic Al Kα X-ray, 650 μm spot size). All of the density functional theory (DFT) simulations were carried out using the Quantum ATK DFT package, 23,24 which is based on a linear combination of numerical atomic orbitals. FHI pseudopotentials with double ζ polarized (DZP) basis set were employed. To accurately account for the long-range van der Waals (vdW) interaction of ZnO and gas molecules, Grimme DFT-D2 van der Waals corrections were utilized. 25 The Perdew−Burke−Ernzerhof (revPBE) exchange−correlation functional was employed, and a 15 Å vacuum distance was used above the ZnO layer to overcome any spurious interactions with the adjacent supercell. A density mesh cutoff of 100 Ha was employed in these simulations.
2.3. Sensor Fabrication and Gas Response. Interdigitated electrode-based sensor devices were fabricated by sputtering 10 nm of titanium and 150 nm of platinum electrodes (10 μm electrode distance) on a silicon oxide thinfilm (300 nm)-coated substrate, followed by photolithography and dry etching process as reported in our previous work. 22 The as-synthesized sensing materials (0.5 wt % dispersed in DIW) were drop-casted on the substrate accompanied by heating on a hot plate (100°C), to evaporate the solvent. The devices were further taken for annealing in a muffle furnace (350°C, 2 h) before exposing to a gas chamber. In-house gas sensing experiments were conducted with oxides of nitrogen, toluene, acetone, and ammonia, which were diluted with 2 L/ min (2000 sccm) of synthetic air adjusted using mass flow controllers (MFCs). The target gas dilution calculations are shown in eq 1 = × + = × + 2.2 ppm 20 ppm flow of target gas total flow of(air target) gas 20 ppm flow of target gas 2000 flow of target gas (1) where flow of target gas is 275 sccm.
The temperature of the gas chamber was varied from room temperature to 300°C, employing a Joule heating system using a ceramic heater adjoined to the power supply. The adjacent electrical resistances and gas sensing responses were obtained and evaluated using a digital multimeter and data acquisition software (FLUKE). The sensor responses were calculated as = S R R 1 g a (oxidizing gas) and = S R R 2 a g (reducing gas), where R a and R g are the resistances of the sensor in air and in the presence of target gas, respectively. The gas sensing measurements were further evaluated by introducing humid air (combination of dry synthetic air with 100% humid air cylinder controlled by MFCs). The final humidity within the chamber was monitored using a humidity sensor (Farnell, T9602-3-D, U.K.).

RESULTS AND DISCUSSION
3.1. Pure and Doped Hf-ZnO Porous Microstructures: Structural Characterization. XRD spectra of the assynthesized pure and Hf-ZnO microstructures are shown in Figure 2. All of the diffraction peaks were indexed well with the crystalline hexagonal wurtzite phase of ZnO (JCPDS No. 36-1451). 26 No peaks corresponding to HfO 2 or mixed oxides were evident in the materials prepared with hafnium doping Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article concentration ≤5%. However, weak intensity peaks corresponding to a HfO 2 phase started to appear in the materials containing a doping concentration of 7% at 2θ = 28.3°c orresponding to the (1̅ 11) plane of cubic HfO 2 (JCPDS No. 83-0808). 27 As the doping concentration increased, the intensity of the peaks at 42 and 51.8°emerged, which are related to the (321) and (022) planes of HfO 2 . 27 This implied that 5% was the critical doping point at which the dopant ions (Hf 4+ ) were well incorporated into the ZnO lattice by substituting the host ions (Zn 2+ ), whereas a secondary phase of HfO 2 starts forming at higher doping concentrations.
Further, this small shift toward a lower 2θ was observed with increasing doping concentration, indicating lattice distortion, owing to the successful substitution of Zn with Hf, thereby causing a decrease in the lattice parameters. Additionally, a minor peak widening was also observed with an increase in doping concentration until 5% Hf-ZnO, suggesting a reduction in the crystallite size. The crystallite size was further estimated using the Scherrer method, as shown in The FE-SEM images of the as-synthesized pristine and Hf-ZnO microstructures are shown in Figure 3. The pure ZnO and 1, 3, and 5% Hf-ZnO (Figure 3a−d) samples are thin nanosheet-assembled microspheres, forming a unique flower morphology, measuring ca. 6−7 μm in diameter.
The detailed growth mechanism of porous nanosheets assembled ZnO microsphere from the precursor is well documented in ref 22, as shown in the below equations It was illuminated that calcination of zinc hydroxide carbonate precursor in different ambient environments, air, or vacuum, brought profound alterations to the morphology and oxygen vacancy content of the ZnO nanostructure, accompanied by the evaporation of H 2 O and CO 2 , bringing in the porous nature of the ZnO morphology. It was evident that calcination in vacuum resulted in higher porosity, smaller particle size, and hence better sensing behavior. Here, further modification of vacuum-annealed ZnO microspheres with hafnium doping showed modulation of the morphology to a whole new extent. It was observed that the morphology of the pristine ZnO ( Figure 3a) was consistent until 5% of Hf doping to ZnO. Although the flower morphology was maintained, the nanosheets appeared to be thinner with increasing doping concentration. The sheets appeared to be the thinnest for the 5% Hf-ZnO microsphere, which can prove to be a very promising material toward NO X gas sensing owing to its larger surface area, defects, and porosity of the sheets (Figure 3d). This is because until 5% doping, the Hf 4+ dopants substituted well into the ZnO matrix without the formation of any secondary oxides and without deforming the lattice structure and only resulted in a decrease in particle size due to charge   Figure 3e. These particles with a high surface energy generally tend to agglomerate, and thus further additional doping with 10% Hf resulted in the agglomeration of the nanoparticles, mostly due to the growth mechanism involving Ostwald's ripening and high surface energy, thereby resulting in adjuration of the particles to grow into caltrop-like microstructures. Studies 28, 29 showed similar morphology alteration owing to the incorporation of dopants. Thus, tuning doping concentration can result in morphology alteration. Further analyses utilizing TEM permitted us to shed light on the comprehensive characteristics of the samples, as shown in Figure 4. For the pristine and 1, 3% Hf-ZnO samples, the nanosheets composed of small nanoparticles carelessly interlaced, forming a large number of pores. The main difference between these samples was the nanoparticle size. Figure 4 represents the statistical size distribution of the samples (inset) by histogram representation. The average particle sizes of the pristine ZnO were 9.5 nm, which was 2 nm bigger than 1 and 3% Hf-ZnO, which had particle sizes of 7.5 and 7.25 nm, respectively. The thin sheets of 5% Hf-ZnO sample seemed to be highly porous with fringes along the edges, and particles of ∼5 nm were observed. The 7% Hf-ZnO sample is composed of quite large, interconnected particles with an average particle size distribution of 52.5 nm, much larger than the pristine ones. Finally, the 10% Hf-ZnO sample is composed of very fine sheets that were closely wound within themselves, acquiring a caltrop-like structure. The inset shows high-resolution images of all of the samples, displaying lattice fringes (d spacing = 0.28 and 0.315 nm), relating to the (002) and (1̅ 11) planes of the wurtzite phase of ZnO and the cubic phase of HfO 2 (Figure 4f), thereby confirming the segregation of HfO 2 . 3030 Mean particle size reduction of the Hf-ZnO microstructures than that of the pristine and the formation of a highly porous morphology at 5% Hf-ZnO contributed toward improved gas sensing response. High porosity and small particle size control the diffusion of gases and surface reactions, as the reduction in particle size increases the active surface area and porosity. From the BET data, the specific surface areas of ZnO and 1, 3, 5, 7, and, 10% Hf-ZnO samples were estimated to be 65.3, 67.8, 69.6, 82.9, 8.9, and 19.02 m 2 / g, respectively. As anticipated, the results exposed that the samples (5% Hf-ZnO) with smaller particle sizes showed the largest specific surface areas.
3.2. Pure and Doped Hf-ZnO Porous Microstructures: Defect Analysis (Oxygen Vacancies). The PL spectra obtained at room temperature for pristine and Hf-ZnO samples are exhibited in Figure 5 (λ ex = 325 nm). A broad green emission (deep level emission, DLE) was detected at 530 nm, which is mostly associated with the defects related to oxygen vacancy (V O ) at the surface of samples. 31,32 No alteration in the position of the peaks and intensity enhancement of the DLE peak with increased doping concentration from 1 to 5 wt % with further reduction in  Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article peak intensity with a higher doping concentration (7−10 wt %) confirmed the variation in defect level within the synthesized materials. The reduction in DLE peak intensity was observed due to enhanced scattering of photons by doping-induced defects. 31 Furthermore, doping resulted in the formation of various defect levels which behave as trapping centers, thereby enhancing the recombination sites. Consequently, the intensity of the green emission band was expected to increase for the sample with a 1−5% Hf dopant, indicating a higher concentration of oxygen vacancies. Hf ions substitute Zn without phase deformation of the host ZnO up to a 5% doping concentration. Hence, enhanced gas sensing properties were observed for 5% Hf-ZnO NPs. Further doping (7−10% Hf) resulted in the collapse of the flower morphology, accompanied by decrease in oxygen vacancy concentration leading to a decrease in DLE peak intensity. 29,31 The XPS spectra of pristine and Hf-ZnO samples are demonstrated in Figure 6. The Zn 2p spectra of all of the samples are shown in Figure 6c. A slight shift of Zn 2p peaks was observed with increasing doping concentration, owing to lattice distortion. 14 Studies already showed a shift in the XPS spectra peaks of doped ZnO samples. 33,34 For instance, a shift of ∼1 eV was observed for the peaks in the XPS spectrum of 2% Co-doped ZnO sample. 35,36 Other reasons associated with this shift have been ascribed to charge transfer, particle size, and lattice strain in nanoparticles. 35 32,37,38 The total areal percentage of each peak was calculated using the Gaussian fitting, which clearly hinted toward an increase in V O concentration with an increase in doping from 1 to 5% and a decrease with further doping (7− 10%). Table 2 Shannon effective ionic radii for Zn 2+ , and Hf 4+ are 0.74 and 0.78 Å, respectively. Because of the similar ionic size, the lattice strain developed by the cation substitution is not significant. Hence, it is evident that the structure of the host lattice is stable even after the formation of atomic defects due to cation substitution. Thus, it is speculated that the V O generation depends strongly on the doped Hf ions' valence state. 39 However, a further rise in doping concentration above a critical concentration leads to the suppression of V O as it induces the precipitation of a HfO 2 -rich phase, resulting in particle agglomeration.  Figure 7a,b. The gas response values linearly increased with increasing NO X concentration, and the response of the 5% Hf-ZnO gas sensor was the highest at all concentrations of gas compared with any other sample. The responses of 5% Hf-ZnO toward 0.2, 0.3, 0.4, 0.6, and 0.8 ppm of NO X at 100°C were 35, 40, 42, 58, and 67, respectively, which were significantly higher than the sensing responses of pristine ZnO (21, 22, 29, 32, and 40). The responses of the 1, 3, and 5% Hf-ZnO gas sensors were higher than the pure ZnO gas sensors, whereas the responses of the 7 and 10% Hf-ZnO samples drastically dropped below the response of the pure ZnO gas sensor. This was attributed to the higher surface area, porosity, and presence of oxygen vacancies in 1−5% Hf-ZnO. The responses of all of the samples were observed at various operating temperatures (25, 50, 100, 150, 200, 250°C). Among all of the samples, the 5% Hf-ZnO gas sensor reached the maximum response (S = 67) at 100°C (Figure 7c). Therefore, all further measurements were performed at 100°C as the response decreases drastically beyond 100°C. For the responses recorded at lower temperatures (<100°C), the rate of reaction of NO X gas with the sensor surface is reduced attributed to inadequate thermal energy. However, at higher operating temperatures (⩾150°C), desorption of gas molecules is relatively faster than adsorption, thereby also decreasing the gas sensing response. Thus, at an optimized temperature of 100°C, both the Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article adsorption and desorption processes are equivalent, exhibiting maximum gas sensing response. It is also worth noticing that room-temperature detection is possible with the 5% Hf-ZnO gas sensor. The reproducibility of the 5% Hf-ZnO gas sensor was explored by measuring the sensor response for repeated cycles over different concentrations. As shown in Figure 7d, a similar response was observed for all of the measurements. The lowest point of detection or limit of detection (LOD) of the 5% Hf-ZnO was evaluated by plotting the linear curve of the gas sensing response versus concentration of the gas and then extrapolating the straight-line portion to the concentration axis at response = 0 (Figure 7e). The LOD was determined to be 22.8 ppb. Furthermore, the sensitivity of the sensor (56.03 ppb −1 ) was estimated from the slope (response vs. concentration). The sensor's sensitivity was significantly improved by doping it with 5% Hf. The response and recovery times as shown in Figure 7f are the two crucial features responsible for sensor's performance evaluation. The times required by a sensor to reach either 90% of its maximum point when exposed to analyte gas or 10% toward the baseline after stopping the gas flow are denoted as response and recovery times. The response/recovery times of the 5% Hf-ZnO gas sensor were 24/26 s, which were significantly faster than the pristine ZnO gas sensor (90/100 s). This showed that the 5% of Hf-ZnO gas sensor was more sensitive to NO X than the pure ZnO gas sensors. Further gas sensing measurements of 5% Hf-ZnO toward 0.8 ppm of NO X were conducted in a humidity-controlled chamber under different RH, ranging from 30 to 70% RH, at 100°C, to check the long-term stability of the 5% Hf-ZnO sensor (Figure 8a). The sensor response remains constant till 40% RH with slight decreases in response (58) at 50% RH and 56 and 55 at 60 and 70% RH. A total of 17% reduction in response is observed under high humidity conditions. This phenomenon is observed due to the presence of water molecules in the air, which gets adsorbed onto the surface of Hf-ZnO, thereby reducing the number of available active sites for the gas interaction. The response is recovered after keeping the sensor in a desiccator for 5 h, hinting toward the reusability of the sensor. Additionally, the long-term stability of the sensor was also verified, as the sensor showed very stable behavior over a period of 65 days (∼2 months), where the sensor was kept in the desiccator and taken out at regular intervals for the test. Finally, gas selectivity, also an important parameter for evaluating gas sensor performance, was investigated for all of the samples toward NO X , ethanol, acetone, and ammonia. The results (Figure 8c) indicated that all of the samples, especially the 5% Hf-ZnO gas sensor, had very good selectivity to NO X , making it feasible to distinguish NO X gas as a specific target gas amidst the mixture.
Thus, a 5% Hf-ZnO gas sensor showed fast and the highest gas response with high stability and selectivity toward NO X gas. Therefore, modulating the Hf doping concentration altered not only the morphological and microstructural properties but also the gas sensing behavior. It is evident from the results that efficient charge transfer between the Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article dopant ion and the host lattice due to charge mismatch (Hf 4+ and Zn 2+ ), high porous morphology (grain size refinement), surface area increments, and strong basic nature of Hf, leading to Lewis acid interaction, which are the main contributing factors for promoting oxygen vacancies formation, result in an increase in the number of adsorption sites. This in turn enhances the gas sensing behavior of 5% Hf-ZnO over the pristine sample. 3.4. Gas Sensing Mechanism with Density Functional Theory (DFT) Validation. Thus, here, we fabricated highly sensitive and selective NO X gas sensor with Hf-doped ZnO. By optimizing the hafnium doping concentration, we modulated the morphology and oxygen vacancy concentration, which enhanced the performance of the sensor almost 2-fold (67 at 100°C toward 0.8 ppm of NO X ) with 5% Hf. It is quite noteworthy to observe that both sensor performance and oxygen vacancy concentration depend on the doping percentage of hafnium. Both response and defects increased with an increase in doping until 5%, beyond which they started declining (10%). Thus, it is well established that oxygen vacancies induced due to doping with Hf played a pivotal role in increasing NO X sensing.
In general, under ambient exposure and depending on the ambient temperature, oxygen molecules, which are adsorbed on the surface of the sensing material, possess different charge states (O 2 − , O − , and O 2− ) due to ionization arising due to the extraction of electrons from the conduction band and hence trapping them onto the surface, thereby resulting in resistance change of the gas sensor. 43 This oxygen-adsorbed surface of ZnO is exposed to the NO X gas, which in turn extracts more electrons by reacting with the adsorbed oxygen species (NO X has an elevated electron affinity of ∼2.28 eV > 0.43 eV than that of oxygen), 44−46 thereby causing a sharp rise in resistance and increasing the depletion layer.
This interaction rises between NO X molecules, and the adsorbed oxygen species increases drastically, leading to higher resistance change, due to the provision of more adsorption sites owing to an increase in a number of oxygen vacancies. Oxygen vacancies or defect-enriched sites are preferential adsorption sites for NO X molecules (41.4 kcal/mol) than the nondefective sites (13.8 kcal/mol), which are higher in concentration in Hf-doped ZnO than in pristine ZnO. From Figure 9, the relation between high gas response at a low temperature and a high oxygen vacancy concentration in 5% Hf-ZnO can also be observed. In addition, the high basicity of the sensing material, due to the addition of hafnium increased the interaction with the NO X molecule, being acidic in nature. Apart from XPS, density functional theory (DFT) was also employed to investigate the impact of oxygen vacancies on the NO X sensing caused by the Hf doping.
The . First, these atomic configurations were geometrically optimized by relaxing the atomic structures until the remaining residual force was smaller than 0.05 eV/Å. During these optimizations, the bottom two layers were fixed in position. In all of these atomic structures, initially, the NO 2 molecule was placed in the same position and orientation above the ZnO slap. These atomic configurations are shown in Figure 10 and Supporting Movies. In the configuration of ZnO without any defect, the oxygen atoms of the NO 2 molecule moved Figure 9. Schematic diagram representing the sensing mechanism of pure and 5% Hf-ZnO microsphere to air and NO X gas environments. To understand the charge transfer between NO 2 molecule and ZnO, Mulliken population analysis was carried out. In the case of ZnO-NO 2 , ZnO-Hf-NO 2 , and ZnO-O vac -NO 2 , a charge of 0.259, 0.113, and 0.217 e, respectively, was transferred from the NO 2 molecule to ZnO. As the topmost layer, oxygen atoms of ZnO lack Zn electron contribution and they take electrons from the NO 2 molecule and the atoms around the oxygen vacancy position rearrange. In the structure with one Hf substitutional with an oxygen vacancy, the NO 2 molecule received 0.288 e from the ZnO-Hf-O vac supercell. This was attributed to the availability of electrons from the Hf atom and the presence of an oxygen vacancy. The electron density difference of the supercells ZnO-Hf, ZnO-Hf-O vac , and ZnO-O vac with respect to the ZnO supercell is plotted in Figure 11.
The case of a supercell with substitutional Hf atom depicts the presence of the extra electron density around the Hf atom ( Figure 11a). Electrons are depleted around the oxygen vacancy position of the ZnO-O vac supercell (Figure 11b). Especially, when the substitutional Hf atom and the oxygen vacancy are present, then the electron density is depleted at the oxygen vacancy position and extra electron density exists around the Hf atom. This combination attracts the NO 2 molecule toward the oxygen vacancy and then leads to stronger bonding. Therefore, the presence of an oxygen vacancy induced by Hf atom doping leads to chemisorption of the NO 2 molecule.
The binding energy of the NO 2 molecule on the supercells of different ZnO is calculated as E Bind = E (ZnO-X/Nb-NO 2 ) − (E ZnO-X + E NO 2 ), in which E (ZnO-X/Nb-NO 2 ) is the total energy of the NO 2 molecule adsorbed on the ZnO-NO 2 , ZnO-Hf-NO 2 , ZnO-Hf-O vac -NO 2 , and ZnO-O vac -NO 2 supercells; E ZnO-X is the total energy of the ZnO, ZnO-Hf, ZnO-Hf-O vac , and ZnO-O vac supercells; and E NO 2 is the total energy of NO 2 molecule. The calculated NO 2 molecule binding energies were 1.31722, 1.08681, 2.87061, and 1.11938 eV for ZnO-NO 2 , ZnO-Hf-NO 2 , ZnO-Hf-O vac -NO 2 , and ZnO-O vac -NO 2 atomic structures, respectively. These binding energies indicate weaker bonding of the NO 2 molecule with the ZnO, ZnO-Hf, and ZnO-O vac materials and stronger bonding of the NO 2 molecule with the ZnO-Hf-O vac material. These results explain the high sensitivity and excellent selectivity observed in the samples with Hf doping (5 wt %) ZnO oxygen vacancies. On the other hand, the binding energy of the oxygen atom to the NO 2 molecule was calculated to be 6.3216 eV, which was much higher than the binding energy of the NO 2 molecule to the ZnO-Hf-O vac material. The atomic distances of the bonded oxygen atom of the NO 2 molecule to ZnO were relatively longer than the oxygen atom distance in ZnO (see Supporting Information Figure S1). Because of this higher binding energy of the oxygen atom to the NO 2 molecule, longer bonding length, high-temperature sensing measurements, and fast response and recovery might be possible.

CONCLUSIONS
Pristine and Hf-ZnO gas sensors were prepared using hydrothermal synthesis. Doping with hafnium played a significant role in modifying the morphology of the porous nanosheet-assembled ZnO flowerlike nanostructure and enhancing the NO X gas sensitivity. Detailed characterization showed successful substitutional doping up to a 5%, which significantly increased the amount of V O (evident from PL and XPS), where XPS showed the highest concentration of V O in 5% Hf-ZnO (78%), then ZnO (63%), and further a particle  Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article size reduction was observed (as confirmed by XRD and TEM: 5 nm for 5% Hf-ZnO and 9.5 nm for pristine ZnO), thereby enhancing the specific surface area (BET: 82.9 m 2 /g for 5% Hf-ZnO and 65.3 m 2 /g for pristine ZnO). DFT analysis indicated that weaker bonding (binding energy) of the NO 2 molecule with the ZnO (1.31722), ZnO-Hf (1.08681 eV), and ZnO-O vac (1.11938 eV) and stronger bonding of the NO 2 molecule with ZnO-Hf-O vac material (2.87061 eV) were the causes of the elevated sensitivity and outstanding selectivity observed in the Hf-doped (5 wt %) ZnO containing more oxygen vacancies. The superior performance of the 5% Hf-ZnO microstructure to NO X gas was discovered. The sensing test fallouts directed that a suitable quantity of Hf doping (5 wt %) significantly upgraded the gas detecting properties (S = 68−0.8 ppm of NO X gas) with elevated sensitivity (22 ppb) and outstanding selectivity, to achieve faster response and recovery times (24/26 s) than pristine ZnO (90/100 s). Hf-ZnO offered outstanding gas sensing behavior because of the presence of oxygen vacancies between host (Zn) and dopant (Hf) lattice. Hence, the gas sensing mechanism of ZnO can be optimized during the synthesis process by increasing oxygen vacancies. A strong Lewis acid−base interaction shows a significant contribution to detect NO X by Hf-doped ZnO. Hafnium doping refines the grain size of ZnO and converts it to highly porous, thin nanosheets, which enhance the NO X gas diffusion throughout the material, which in turn increased the number of adsorption sites and the surface area, and finally, the NO X gas response is increased significantly. However, higher doping (≥7 wt %) showed deterioration of gas detecting properties of ZnO and destruction of the morphology owing to the formation of HfO 2 , which also suppressed the oxygen vacancy content and decreased the specific surface area.
■ ASSOCIATED CONTENT
Nearest atomic distances between the ZnO-bonded oxygen atom of NO 2 molecule and ZnO atoms and the oxygen atom in ZnO and atomic configurations of four different types of supercells (PDF) Figure S2